1. Field
The following disclosure relates to a color tunable multifunctional nanophosphor, a synthesis method thereof, and a polymer composite including the nanophosphor, and particularly, to a nanophosphor capable of being implemented by tuning various colors of green, yellowish green, yellow, orange, red and the like under a single wavelength excitation in a manner of adjusting doping elements, and a synthesis method thereof. The nanophosphor may also be nanophosphors which are capable of being used as contrast agents of magnetic resonance imaging by forming a shell, and also being applied to transparent displays in a form of polymer composite.
2. Background
Nanophosphors have a structure that lanthanide is doped on an oxide, fluoride, sulfide or nitride-based host material with a size less than 100 nm. A nanophosphor doped with trivalent lanthanide ions except for Ce3+ ion represents an inherent luminescent color depending on the doped lanthanide element, irrespective of a type of host [luminescent materials (1994)]. This is because the photoluminescence (luminescence) of the nanophosphor is generated by 4f-4f transition due to 4f electrons of the doped lanthanide 3+ ions. Therefore, a desired photoluminescence wavelength may be advantageously maintained even if sizes of particles are differently adjusted when necessary.
However, since the nanophosphors exhibit fixed luminescent colors, such as red or green, according to doped elements, they have difficulties in obtaining desired luminescent colors, except for several fixed colors. To overcome this problem, upon mixing two or more types of phosphors, which have different luminescent colors, various colors can be implemented but excitation light sources with different wavelengths have to be disadvantageously used. This may result from the fact that each element has a specific absorption wavelength range for photoluminescence via 4f-4f transition, and accordingly the mixed phosphors do not all emit light under a single excitation wavelength.
To solve this problem, Wang, et al. obtained photoluminescence properties of green, red and bluish green colors using a single wavelength around 250 nm in a manner of coating Ce on NaGdF4 particles doped with Tb, Eu, Sm and Dy, respectively, to absorb ultraviolet rays around 250 and transferring the absorbed energy to each lanthanide element [Nanotechnology vol. 18, 025701 (2007)]. However, in this case, only a green-emitting photoluminescence property was obtained when Ce and Tb were co-doped, only a red-emitting photoluminescence property was obtained when Ce and Eu were co-doped, only the red-emitting photoluminescence property was also obtained when Ce and Sm were co-doped, and a bluish-emitting photoluminescence property was obtained when Ce and Dy were co-doped. That is, only an inherent luminescent color emitted by each of Tb, Eu, Sm and Dy was realized. Also, a less amount of the co-doping agent Ce lowers adsorption efficiency of excitation light. This may cause a difficulty in obtaining a strong photoluminescence property.
Also, when using photoluminescence properties of individual nanophosphors, such as bio imaging contrast agents, mixing of various types of nanophosphors for use may not be allowed in some cases. In general, organic to dyes are widely used as the bio imaging contrast agents. The organic dyes have characteristics of representing various luminescent colors and exhibiting high photoluminescence intensity according to types. However, due to extremely low photostability, a slight increase in an exposure time to excitation light may cause drastic lowering of the photoluminescence intensity [ACS Nano vol. 6, 3888-3897 (2012)].
To overcome the problem, attempts are made to apply inorganic luminescent materials, such as quantum dots. However, the quantum dots cause photoblinking of light [Nature vol. 459, 686-689 (2009)], and are difficult to be applied upon containing a heavy metal, such as Cd, for example, containing CdSe.
Therefore, it is seriously necessary to develop a new material which facilitates for luminescent color tuning, and exhibits high photostability without photoblinking. If a nanophosphor which can emit light in various wavelength bands under one excitation wavelength is created, the nanophosphor may be applied as bio imaging contrast agents capable of detecting different materials, and enable the implementation of a display device with ultrahigh image quality, capable of representing various colors.